For improvement in the power generation efficiency of a steam turbine power plant, it is effective to increase the main steam temperature. Conventional steam turbine power plants with a main steam temperature of 600° C. or more are commercially in operation and the development of a steam turbine with a main steam temperature of 650° C. or so is underway. For a higher efficiency, efforts to develop a steam turbine with a main steam temperature of 700° C. or more have also been continued.
For a steam turbine to achieve a main steam temperature of 700° C. or more, a rotor of the turbine should be made of Ni-base super alloy instead of steel which has been so far used as the rotor material because the allowable temperature limit of steel is about 650° C. Ni-base super alloy has higher strength than steel but it is more expensive and it is difficult to make a large forging such as a rotor from it. Currently Ni-base super alloys suitable for the manufacture of large forging are being developed, evaluated and tested and some of these Ni-base super alloys are expected to be usable for the manufacture of 10-ton class forging. Nevertheless, the weight of rotors used in common large steam turbines is in the range of 30-40 tons. For this reason, a welded rotor which is manufactured by welding several forged members is being explored. Furthermore, a compact high-temperature turbine of the top turbine type which comprises only parts to be exposed to high temperatures has been conceived (for example, see E. Saito, et al., “Development of the Ultra-Supercritical Steam Turbine for Large Coal-fired Power Plants”, Proc. Power-Gen International, (2004)).
For a steam turbine plant to realize a main steam temperature of 700° C. or more, it is essential to provide not only a technique of manufacturing high temperature resisting parts of the turbine and boiler from Ni-base super alloy but also a technique of reducing the use of Ni (more expensive than steel) and expensive Co and Mo contained in some Ni-base super alloys.
In the case of welded rotor type turbines, provided that all rotors of 30-40 tons are made of Ni-base super alloy, a large quantity of nickel will be required; therefore, members to be exposed to high temperatures (a total weight thereof is equivalent to about 10 tons) are made of super alloy and the other members to be used at low temperatures are made of a steel material such as 12Cr steel and these members are joined. Since the total weight of the members for use at high temperatures is at most 10 tons or so, the use of Ni is reduced.
In this case, however, it is difficult to ensure reliability for the following reason: as the assembly of members of different materials is used for a long time at high temperatures, the compositional difference between materials may cause diffusion of elements, resulting in deterioration of the joints. Another problem is as follows: ferrite steel is used as the steel material because of its high strength at high temperatures and Ni-base super alloy has a thermal expansion coefficient higher than ferrite steel and this difference in thermal expansion coefficient may cause cracking due to thermal stress in the joining process or fatigue fracture due to thermal stress in use. For this reason, a Ni-base super alloy with a low thermal expansion coefficient must be used.
Among Ni-base super alloys are Ni—Fe-base super alloys which contain much Fe but have a high strength. However, Ni—Fe-base super alloys are not suitable as materials for welded rotors because Fe increases the linear expansion coefficient. In order to make the linear expansion coefficient of Ni-base super alloy almost equal to that of ferrite steel, the use of Fe (though inexpensive) should be avoided and the percentage of Mo, which decreases the thermal expansion coefficient, should be increased.
Ni-base super alloy which contains much Mo is suitable as a material for welded rotors but is costly because it does not contain inexpensive Fe but contains much Mo, a material more expensive than Ni. In case of the top turbine type, reliability is high because of the absence of welded portions and Ni—Fe-base super alloy, less costly, can be used, but the need for an additional turbine leads to cost rise.
Next, the problem with the boiler which produces high-pressure steam for supply to the steam turbine will be described.
In a large-scale steam turbine plant, the height of the boiler is generally 70 m or more and its higher portions is heated to higher temperatures than its lower portions and the piping for high-temperature high-pressure steam to be supplied to the steam turbine extends from the top of the boiler to the turbine building on the ground with its total length of 100 m or more.
For a steam turbine plant with a main steam temperature of 675° C. or more, since the allowable temperature limit of steel is 650° C. or so, the piping for high-temperature high-pressure steam must be made of Ni-base super alloy. This steam piping has an outside diameter of approximately 600 mm, a wall thickness of approximately 100 mm and a total length of 100 m or more and thus its total weight is much larger than the weight of Ni-base super alloy used in the turbine.
For a main steam temperature of 700° C. or less, the boiler material may be a Ni—Fe-base super alloy which is advantageous in terms of cost and workability, such as HR6W; however, for a main steam temperature of more than 700° C., the material should be a solution-hardened Ni-base super alloy with a high strength such as IN617 and for a main steam temperature of 720° C. or more, it should be a precipitation-hardened Ni-base super alloy with a higher strength such as Nimonic 263. Since IN617 and Nimonic 263 are not only costly but also poor in workability, it is impossible to manufacture a long pipe with an outside diameter of 600 mm or so from these super alloys. Therefore, a plurality of pipes with a smaller outside diameter must be used to supply high-temperature high-pressure steam from the boiler building to the turbine building but the use of plural pipes means an increase in weight per flow area and an increase in the total piping weight, leading to further cost rise.
With this background, efforts to develop a horizontal boiler which replaces the vertical boiler have been made for the purpose of shortening the piping between the turbine building and boiler building. The problems with a horizontal boiler are deterioration in combustion efficiency and a substantial increase in required installation space.
An object of the present invention is to provide a high temperature steam turbine power plant which uses a vertical boiler with a high combustion efficiency to achieve a main steam temperature of 675° C. or more and a power output of 100 MW or more and ensures both reliability and cost reduction.